Hydrocarbon fuel with improved laminar burning velocity and method of making

ABSTRACT

A hydrocarbon fuel such as a gasoline exhibiting substantially improved laminar burning velocity and method of making. The hydrocarbon fuel may comprise a paraffinic fraction, an olefinic fraction, and an aromatics fraction. The aromatics fraction may comprise methyl aromatics and non-methyl alkyl aromatics wherein the percentage of non-methyl alkyl aromatics in the aromatics fraction is at least 30% by volume. The fuel may comprise a methyl aromatics fraction comprising xylenes wherein the percentage of ortho- and para-xylene in the xylene fraction is at least 60% by volume.

This application claims the benefit of U.S. Ser. No. 60/485,001 filedJul. 3, 2003.

FIELD OF THE INVENTION

The present invention relates to an improved. hydrocarbon fuel andmethod for making it. More specifically, it relates to a hydrocarbonfuel exhibiting improved laminar burning velocity. The improvedhydrocarbon fuel substantially increases engine efficiency.

BACKGROUND OF THE INVENTION

Increasingly more stringent emissions and efficiency regulations pose asignificant hurdle to internal combustion engine makers. Current sparkignition and compression ignition engine efficiencies are well below thetheoretical maxima, and even small efficiency improvements are highlydesirable. Many engine makers are developing sophisticated hardwarecontrols to extract more efficiency from the combustion cycle. Forexample, techniques such as direct injection, homogeneous chargecompression ignition, variable valve timing, and turbocharging have beencommercialized to varying levels, and have proved successful inimproving efficiency. The effects of fuel composition on engineefficiency have also been actively studied. Presently, a fuel's octanenumber is considered to have the most significant impact on engineefficiency, since higher octane number fuels allow a closer approach tooptimum spark advance timing and permit increased compression ratiooperation. The effects of the fuel's laminar burning velocity (or theclosely related laminar flame speed) on engine efficiency have also beenstudied but are not as well understood. It is generally recognized thatfaster burn rates in engines lead to higher efficiency. For this reasonthere has been a trend in engine designs in recent years to modify themechanical design of the fuel system and/or combustion chamber (e.g.,increased swirl and/or tumble) to enhance burn rates. Engine correlationtools developed to predict burn rates traditionally incorporate thefuel's laminar flame speed (SAE800133). Further, it has been shown thatincreases in engine burn rates in a modern lean burn type enginecorrelate directly with increases in fuel laminar flame speedmeasurements made in a constant volume combustion chamber (U.S. Pat. No.6,206,940). However, laminar flame speeds or burning velocities of fullyblended fuels are not typically measured, nor are they readily estimatedthrough surrogate analytical techniques. Whereas standardized octanemeasurements have been carried out and consistent data acquired for alarge fraction of the hydrocarbons commonly found in commercialgasolines, the same is not true for burning velocities, and consequentlythe effects of fuel composition on burning velocity are not wellunderstood.

Several approaches have been investigated to boost the burning velocityof a fuel. One approach is to add an additive not normally present incommercial gasoline streams. For example, U.S. Pat. No. 5,354,344 A1describes a gasoline fuel composition containing 5-50% by volume of thechemical 3-butyn-2-one. This additive is said to improve the flamepropagation speed, engine output power, ignitability, and reducecycle-to-cycle fluctuations, although no assertions are made related toimproving vehicle efficiency. However, because this additive is a purechemical component that requires a multi-step chemical synthesis, itsintroduction into commercial gasolines at the claimed dosages wouldinvolve significant expense, and it is doubtful that the resulting fuelcould be made widely available.

U.S. Pat. No. 2,894,830 describes the use of small amounts of boronhydrides in conventional fuels employed for heating or propulsionpurposes to increase the combustibility and the velocity of flamepropagation of such fuels.

WO 96/40844 A1 and WO 95/33022 A1 describe the introduction oftransition metals, alkaline metals, alkaline earths, halogens, groupIIIA elements and mixtures thereof into a fuel to increase the fuel'scombustion rate. U.S. Pat. No. 4,765,800 discloses that alkali metalsalts or alkaline metal earth salts of succinic acid derivatives improvethe ignitability of a mixture and shorten flame travelling time. Oneserious drawback of these approaches is the corresponding emission ofuncommon and undesirable pollutants such as boron compounds, metals, orhalogens, which could foul engine/exhaust aftertreatment systems andwould likely require complex aftertreatment controls to reduceenvironmental contamination.

An approach to increase the laminar burning velocity of a fuel thatforgoes the use of additives is to modify its bulk chemical composition.FIG. 1 shows data from four literature sources that measured laminarburning velocities for a wide range of molecules. The data are fromWagner and Dugger, JACS 77:227 1955, Gibbs and Calcote, J. Chem. Eng.Data, 4:226 1959, Albright, Heath, and Thena, Industrial and EngineeringChemistry 44 10 1952, pp. 2490-1496, and Gerstein, Levine, and Wong, J.Am. Chem. Soc., 73:418 1951. As can be seen from the data, burningvelocities are available for only a small number of aromatics.Furthermore, the data are contradictory. For example, Albright et alreport ethylbenzene to be the fastest aromatic while Wagner and Duggerreport benzene to be the fastest. The paucity of experimentalobservations and uncertainties in the data render it difficult toelucidate fuel structure effects from these studies. Thus, while thereis a general understanding in the art on how fuel structure affectsburning velocity for paraffins and olefins, no such understanding existsfor aromatics. The present invention resulted from a thoroughinvestigation of the burning velocity for a wide range of aromaticcomponents, from which we have found that the fuel structure effects ofaromatics are actually different from that taught in the art. Theresulting improved understanding makes the optimization of burningvelocity by tuning fuel composition possible.

SUMMARY OF THE INVENTION

The present invention is directed to an unleaded hydrocarbon fuel suchas a gasoline boiling range fuel comprising a paraffinic fraction, anolefinic fraction, and an aromatics fraction having an improved laminarburning velocity. The aromatics fraction comprises methyl aromatics andnon-methyl alkyl aromatics and the percentage of non-methyl alkylaromatics in the aromatics fraction is at least 30% on a volume basis.Preferably, the paraffinic fraction is in an amount of 90% or less, theolefinic fraction is in an amount of 30% or less, and the aromaticsfraction is in an amount of 70% or less, all calculated on a volumebasis. Unless otherwise stated, all percentages listed herein are on avolume basis. The term “paraffinic” as used herein refers to normal,iso, and cycloparaffins, and the term “olefinic” as used herein refersto linear, branched, and cyclo-olefins. The components denoted“non-methyl alkyl aromatics” include molecules such as ethylbenzene,propylbenzene, butylbenzene, and the like, in which a single alkyl chaincontaining two or more carbons is bonded to the aromatic ring. Thecomponents denoted “methyl aromatics” include aromatic molecules such astoluene, o, m, and p-xylenes, trimethylbenzenes, methyl ethylbenzenes,and the like. Components such as oxygenates, di-olefins, benzene, otheraromatics and naphthoaromatics may also be included in the hydrocarbonfuel.

The hydrocarbon fuel preferably contains benzene in an amount less than1% by volume and sulfur less than 30 ppm by weight.

The invention is also directed to an unleaded hydrocarbon fuelcomprising a paraffinic fraction, an olefinic fraction, and an aromaticsfraction, wherein said methyl aromatics fraction comprises xylenes(dimethyl benzenes) and the percentage of ortho- and para- substitutedxylenes is at least 60% on a volume basis.

The invention further relates to a method for making a hydrocarbon fuelsuch as unleaded gasoline, low sulfur gasoline, and low benzene gasolinehaving an improved laminar burning velocity. The terms laminar burningvelocity and laminar flame speed are often used interchangeably in theliterature and this practice will be followed herein.

The method comprises providing a hydrocarbon fuel having a paraffinicfraction, an olefinic fraction, and an aromatic fraction. The aromaticfraction may comprise methyl aromatics and non-methyl alkyl aromatics.The method includes controlling the concentration of the non-methylalkyl aromatics in the aromatics fraction to at least 30% by volume. Yetanother aspect of the invention is directed to controlling thepercentage of ortho- and para-xylenes in the xylene fraction to at least60% by volume. The paraffinic fraction may comprise normal (linear),branched (iso), and cyclo-paraffins, the olefinic fraction may compriselinear, branched, and cyclo-olefins.

The present inventive method and hydrocarbon fuel are advantageous overconventional methods and fuels. Specifically, inventive fuelcompositions exhibit increased laminar burning velocities andsubstantially improved engine thermal efficiencies. A substantiallyimproved thermal efficiency as this term is used in this invention meansa relative brake thermal engine efficiency of at least 0.5%, preferablyat least 1.5% and most preferably at least 2% greater than the brakethermal efficiency obtained with an unmodified conventional fuel.Likewise, a substantially improved burning velocity as this term is usedin this invention means a burning velocity of at least 4%, preferably atleast 10% and most preferably at least 15% greater than the burningvelocity of an unmodified conventional fuel.

Another advantage of the inventive hydrocarbon fuel composition is thathigher burning velocities also improve lean burn engine operation. Leanburn engines are generally known to improve engine efficiency butconventional gasoline blends often burn too slowly to allow a maximumbenefit to be extracted. The burning velocity benefits identified in thepresent invention apply over substantially the entire fuel/airstoichiometry range, i.e., they are not limited to one operating regimesuch as stoichiometric, lean, or rich operation. As such, they areuseful in extending the lean limit of engine operation therebyincreasing engine efficiency. Additionally, the faster heat releaseprovided by fast burning fuels maximizes the power and/or torque outputof the engine. A significant improvement of torque output enabled by afuel composition could allow engine downsizing and thus recoveradditional efficiency benefits from reduced vehicle weight.Additionally, the inventive compositions have the significant advantagethat they can be produced from refinery streams and thus have thepotential of being supplied in large quantities at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the peak burning velocities of several aromatic hydrocarbonspecies as reported in Wagner and Dugger, JACS 77:227, 1955, Gibbs andCalcote, J. Chem. Eng. Data, 4:226 1959, Albright, Heath, and Thena,Industrial and Engineering Chemistry, 44:2490 1952, and Gerstein,Levine, and Wong, J. Am. Chem. Soc., 73:418 1951.

FIG. 2 shows a schematic representation of a constant volume combustionvessel used for laminar burning velocity determinations. A) opticalarrangement; B) simplified gas diagram.

FIG. 3 shows peak burning velocity data for several aromatic hydrocarboncompounds in the gasoline boiling point range, acquired at T=450K andP=3 atm.

FIG. 4 shows laminar burning velocity data at T=450 K and P=3 atm forseveral aromatic species as a function of equivalence ratio φ.

FIG. 5 shows laminar burning velocity data at T=450 K and P=3 atm fortwo fast fuel formulations compared to a reference gasoline according toone embodiment of the present invention.

FIG. 6 shows the relative burning velocities at T=450 K and P=3 atm fora fast and slow fuel blend according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The laminar burning velocities of more than 30 hydrocarbons weremeasured in a constant volume combustion vessel under temperatures andpressures that approximate in-cylinder conditions. The apparatus isshown schematically in FIG. 2. The measurements were carried out in astainless steel vessel with a 16.5 cm diameter spherical cavity(volume=2.4 liter) with four windows for optical access. The vessel washoused in a temperature-controlled oven with quartz windows to transmitSchlieren and ignition laser beams. Liquid fuels were pre-vaporized inan 11 liter stainless steel vessel housed outside of the oven. Thevaporized fuel mixture was metered into the combustion vessel using ahigh sensitivity pressure transducer (PT). Measurements were made over avery wide range of equivalence ratios to characterize the burningvelocity dependence on stoichiometry. The symbol φ denotes the fuel/airequivalence ratio, wherein a value of φ=1.0 represents a stoichiometricfuel/air mixture. The air charge was admitted next until the desiredpressure was achieved. The mixtures were ignited in the center of aspherical vessel with a laser pulse at an initial temperature of 450 Kand initial pressure of 3 atm. The data were acquired over astoichiometry range of a fuel to air ratio (Φ) of from about 0.55 toabout 1.30 to determine how the fuel to air ratio affects the burningvelocity. Following ignition, the pressure rise in the vessel ismonitored with a fast, high dynamic range pressure transducer.

The data of pressure as a function of time data were converted via athermodynamic analysis to mass fraction burned based on the establishedapproach described by Metghalchi and Keck [Metghalchi, M. and Keck, J.C.; “Burning velocities of mixtures of air with methanol, isooctane, andindolene at high pressure and temperature”, Combustion and Flame, 1982,vol. 48, pp. 191-210]. Data from the pressure-based measurements areextrapolated back to the initial conditions (450 K and 3 atm) to ensurethat the fuels are compared under the same temperature and pressureconditions. This method utilizes data in which the flame radius is muchgreater than the flame thickness, rendering the effects of stretchnegligible. The results for ethane and butane acquired under ambientconditions (300 K and 1 atm), for which accurate literature data areavailable for comparison, were obtained for the purpose of validatingthe techniques used herein for accurately determining burn velocity.

The results show that, of the fuels studied, methane is the slowestparaffin and ethane the fastest. Generally, olefins have a fasterburning velocity than the corresponding paraffins. By correspondingparaffin we mean a paraffin that has the same carbon connectivity as agiven olefin, e.g., iso-butene and iso-butane, 2,2,4 trimethyl pentaneand 2,4,4-trimethyl-1-pentene, etc. Branched paraffins are slower thannon-branched (linear) paraffins, and branched olefins are slower thannon-branched (linear) olefins. Aromatics other than benzene aregenerally slower than the olefins and paraffins, while oxygenates arefaster.

The burning velocities of aromatics are illustrated in FIG. 3. As shown,methyl benzenes such as the xylenes and trimethylbenzenes are slowerthan the non-methyl alkyl aromatics such as ethylbenzene andpropylbenzene. Moreover, FIG. 3 shows that among the multi-methylaromatics such as the xylenes and trimethyl benzenes, in which more thanone methyl group is substituted on the aromatic ring, the sites ofmethyl substitution influence burning velocity, that is, ortho- andpara-substituted isomers have a higher burning velocity than themeta-substituted isomers.

One aspect of the present invention relates to a method for blending afuel such as gasoline to increase laminar burning velocity. Such ablended fuel will yield benefits in any engine (either spark ignition,compression ignition, or a combination thereof) in which flamepropagation is operative in consuming the fuel. Generally the methodincludes controlling the composition of the aromatic component of thefuel as taught herein. We have found that the laminar burning velocityof a fuel increases with the following general changes: a) increasingthe concentration of non-methyl alkyl aromatics and decreasing theconcentration of methyl aromatics, and b) increasing the concentrationof ortho- and para-substituted multi methyl aromatics.

One embodiment of the invention increases the laminar burning velocityof a full-range gasoline by altering the composition in such a way as toincrease the concentration of “preferable” compounds and decrease theconcentration of “less preferable” compounds, while keeping the overallpercentage of olefins, paraffins, and aromatics unchanged. The term“preferable compounds” means compounds that, according to the teachingof this invention, increase the fuel's burning velocity. For example,one embodiment of the invention includes keeping the total concentrationof aromatics in the fuel constant while increasing the ratio ofnon-methyl alkyl aromatics in the aromatic fraction, such asethylbenzene, n-propylbenzene, iso-propylbenzene, and t-butylbenzene,and/or decreasing the methyl aromatics such as toluene, xylene, andtrimethylbenzenes. It has been discovered that the variation in burningvelocity between the fastest and slowest aromatics in the gasolineboiling range is about 50%, which is higher than the variations observedamong olefins and paraffins in this boiling point range.

Engine and vehicle data obtained indicate that these modifications cantranslate into a substantial thermal efficiency improvement of at leastabout 0.5%, preferably at least about 1.5%, and more preferably at leastabout 2%. For example, according to one embodiment of the invention twofuels with laminar burning velocities that differ by 1% yield a 2%difference in the relative brake thermal efficiency in an engine test.

Thus, according to the present invention, a fuel's burning velocity canbe increased by increasing the proportion of non-methyl alkyl aromaticsto methyl aromatics, and increasing the proportion of ortho- andpara-xylene to m-xylene.

As shown in FIG. 4, the relative ranking of the aromatics persists toboth lean and rich conditions, meaning that the improvements in burningvelocity achieved by varying the fuel composition may be realized acrossthe entire load-speed operating map of the engine. Stated alternately,since there are no discernible differences between the fuels as afunction of fuel/air ratio φ, that is, the relative differences betweenthe fuels are effectively the same under lean, stoichiometric, and richconditions, there is no basis for defining preferential composition foronly a given part of the drive cycle based on flame speed differences.

An embodiment of the present invention relates to a hydrocarbon fuelcomprising a paraffinic fraction in an amount of 90% or less, anolefinic fraction in an amount 30% or less, and an aromatics fraction inan amount of 70% or less, wherein said aromatics fraction comprisesmethyl aromatics and non-methyl alkyl aromatics and the concentration ofnon-methyl alkyl aromatics in said aromatics fraction is at least 30%.Preferably, the concentration of non-methyl alkyl aromatics in thearomatics fraction may be at least 50%, and more preferably at least70%.

In another preferred embodiment, the methyl aromatics fraction comprisesxylenes and the percentage of ortho- and para-xylene in said xylenefraction is at least 60%. Preferably, the concentration of ortho- andpara-xylene in said xylene fraction may be at least 75%, and morepreferably at least 90%.

The present invention also relates to a method for making a hydrocarbonmixture in the gasoline boiling point range having an improved laminarburning velocity. The method comprises providing a gasoline comprising aparaffinic fraction, an olefinic fraction, and an aromatic fraction. Theparaffinic fraction comprises linear, branched, and cyclo-paraffins, theolefinic fraction comprises linear, branched, and cyclo-olefins, and thearomatic fraction comprises methyl aromatics and non-methyl alkylaromatics. The method further comprises controlling the concentration ofthe non-methyl alkyl aromatics in the aromatics fraction to at least 30%and the percentage of ortho- and para-substituted xylene in the xylenefraction to 60%.

A preferred embodiment comprises controlling the percentage ofnon-methyl alkyl aromatics in the aromatics fraction to at least 50% andthe percentage of ortho- and para-substituted xylene in the xylenefraction to 75%.

A most preferred embodiment comprises controlling the percentage ofnon-methyl alkyl aromatics in the aromatics fraction to at least 70% andthe percentage of ortho- and para-substituted xylene in the xylenefraction to 90%. These and other embodiments of the invention willbecome more apparent to those skilled in this art from the followingexamples.

It has been found that higher burning velocity correlates with increasedefficiency in vehicle tests. Data have been obtained with a prototypevehicle (4-speed ATM, IW=1360 kg) with a 4-cylinder, direct injectiongasoline engine. The vehicle was evaluated with a U.S. driving cycle inwhich lean-burn operation was achieved for half the drive cycle.Multiple test fuels, and a base fuel were evaluated in which thearomatics level, olefin level, and volatility were varied. Laminarburning velocity measurements show that there was about an 11% variationin burning velocity which resulted in about a 2% relative efficiencydifference in the vehicle.

EXAMPLE 1

Two model fuels were blended to have a RON and boiling pointdistribution comparable to a conventional U.S. gasoline. The molecularcomponents were chosen on the basis of maximizing where possible thosemolecules which have an elevated burning velocity. The fuel composition(all values in weight %) and properties are shown in Table 1. TABLE 1Fuel FF1 Fuel FF2 REF Gasoline 1-hexene 23.43 3.50 cyclohexane 9.10methylcyclohexane 5.35 4.99 iso-octane 31.79 1-pentene 5.35 4.163-heptene 3.90 16.35 ethylbenzene 30.18 11.26 1.90 toluene 50.64 8.32 c6isoparaffins 8.79 c7 isoparaffins 6.68 c9 aromatics 6.54 c5 isoparaffins6.07 c5 olefins 5.37 c11 naphthenes 4.84 n-pentane 4.81 c8 olefins 4.55n-hexane 4.17 c10 aromatics 4.03 c11 aromatics 3.23 c11 aromatics 3.23m-xylene 3.13 c8 isoparaffins 2.37 c6 olefins 2.37 c4 olefins 2.11butane 1.72 c7 olefins 1.50 o-xylene 1.47 n-heptane 1.46 p-xylene 1.13n-octane 0.63 sum 100.0 100.0 90.4¹ RON 92.1 92.8 89.8¹The large number of remaining components are present at very smallconcentrations (<1% each) and are not shown.

The burning velocity data for these fuels and a conventional referencegasoline (REF gasoline) are shown in FIG. 5. It can be seen that themolecular constituents can be preferentially chosen to significantlyenhance the burning velocity of the fuel.

EXAMPLE 2

Two fuel blends were prepared containing a single aromatic, olefinic,and paraffinic component. Blend one was composed of iso-octane,2,4,4-trimethyl-1-pentene, and m-xylene, which are a “slow” paraffin,olefin, and aromatic, respectively. Blend 2 was composed of n-pentane,1-hexene, and iso-propylbenzene, which are a “fast” paraffin, olefin,and aromatic, respectively. The concentrations of the paraffin, olefin,and aromatic were chosen to approximate those in commercial gasoline.The compositions of these fuels are shown in the table below. TABLE 2Aromatic Methyl/Non-Methyl Vol % Vol % Alkyl Component Fuel 1 Fuel 2Fuel 1 Fuel 2 n-pentane 60 iso-octane 60 1:0 1-hexene 102,4,4-trimethyl-1-pentene 10 0:1 isopropyl benzene 30 m-xylene 30 Total100 100

The burning velocities of these fuels were determined at 450° K and 3atm. The results, shown in the FIG. 6, show that a burning velocityincrease of 17% was achieved solely by replacing the “slow” paraffins,olefins, and aromatics with “fast” analogues. Thus, preferentiallytailoring the molecular structure of paraffins, olefins, and aromatics,without changing the bulk concentration of these constituents, increasesburn rate, and by extension, engine efficiency.

1. An unleaded hydrocarbon fuel comprising a paraffinic fraction, anolefinic fraction, and an aromatics fraction, wherein said aromaticsfraction comprises methyl aromatics and non-methyl alkyl aromatics andthe percentage of non-methyl alkyl aromatics in said aromatics fractionis at least 30% by volume.
 2. The hydrocarbon fuel of claim 1, whereinsaid paraffinic fraction is in an amount of 90% or less by volume, saidolefinic fraction is in an amount of 30% or less by volume, and saidaromatics fraction is in an amount of 70% or less by volume.
 3. Thehydrocarbon fuel of claim 1, wherein the percentage of non-methyl alkylaromatics in said aromatics fraction is at least 50% by volume.
 4. Thehydrocarbon fuel of claim 1, wherein the percentage of non-methyl alkylaromatics in said aromatics fraction is at least 70% by volume.
 5. Thehydrocarbon fuel of claim 1, wherein said methyl aromatics fractioncomprises xylenes and the percentage of ortho- and para-xylene in saidxylene fraction is at least 60% by volume.
 6. The hydrocarbon fuel ofclaim 1, wherein said methyl aromatics fraction comprises xylenes andthe percentage of ortho- and para-xylene in said xylene fraction is atleast 75% by volume.
 7. The hydrocarbon fuel of claim 1, wherein saidmethyl aromatics fraction comprises xylenes and the percentage of ortho-and para-xylene in said xylene fraction is at least 90% by volume. 8.The hydrocarbon fuel of claim 1, further comprising benzene in an amountof 1% or less by volume, and sulfur in an amount of 30 ppm or less byweight.
 9. An unleaded hydrocarbon fuel comprising a paraffinicfraction, an olefinic fraction, and an aromatics fraction, wherein saidaromatics fraction comprises a xylene fraction and wherein thepercentage of ortho- and para-xylene in said xylene fraction is at least60% by volume.
 10. The hydrocarbon fuel of claim 9, wherein thepercentage of ortho- and para-xylene in said xylene fraction is at least75% by volume.
 11. The hydrocarbon fuel of claim 9, wherein thepercentage of ortho- and para-xylene in said xylene fraction is at least90% by volume.
 12. The hydrocarbon fuel of claim 9, further comprisingbenzene in an amount 1% or less by volume, and sulfur in an amount of 30ppm or less by weight.
 13. A method for making a hydrocarbon fuel havingan improved laminar burning velocity the method comprising: providing ahydrocarbon fuel comprising a paraffinic fraction, an olefinic fraction,and an aromatic fraction wherein said aromatic fraction comprises methylaromatics and non-methyl alkyl aromatics; and controlling the percentageof said non-methyl alkyl aromatics in said aromatics fraction to atleast 30% by volume.
 14. The method of claim 13, further comprisingcontrolling the percentage said of non-methyl alkyl aromatics in saidaromatics fraction to at least 50% by volume.
 15. The method of claim13, further comprising controlling said percentage of said non-methylalkyl aromatics in said aromatics fraction to at least 70% by volume.16. A method for making a hydrocarbon fuel having an improved laminarburning velocity the method comprising providing a hydrocarbon fuelcomprising a paraffinic fraction, an olefinic fraction, and an aromaticfraction comprising methyl aromatics and non-methyl alkyl aromatics,wherein said methyl aromatics fraction comprises xylenes; andcontrolling the percentage of ortho- and para-xylene in the xylenefraction to at least 60% by volume.
 17. The method of claim 16, furthercomprising controlling the percentage of ortho- and para-xylene in thexylene fraction to at least 75% by volume.
 18. The method of claim 16,further comprising controlling the percentage of ortho- and para-xylenein the xylene fraction to at least 90% by volume.